1. Field of the Invention
The invention relates to a partitioning method for use in proximity effect correction in electron-beam lithography.
2. Description of the Prior Art
In the technique of electron beam lithography an electron beam is used to delineate the features of a semiconductor device by selectively irradiating a substrate coated with an electron-beam sensitive resist. The electron beam is deflected and shaped in a precise manner to define the required shape in the resist. The pattern is then developed in the resist. The substrate can be either a semiconductor wafer, as in the case of direct-write lithography, or glass, metal or other material as in the case of a high resolution mask for subsequent use in fabricating the semiconductor device.
Scattering of the electron beam from the resist and the substrate gives rise to non-uniform exposure of the resist and thus to distortions in the developed shapes. This is termed the proximity effect and must be corrected for in an e-beam lithography process either by changing the shapes to be developed or by varying the dose over the design to take the scattering into account.
Since the dose applied to resist may vary over the design, the design pattern must, during the process of determining and producing control signals for the E-beam writing tool, be divided into a number of shapes over which the dose applied by the writing tool is constant. This process is referred to as shape partitioning.
The prior art approaches to shape partitioning for E-beam lithography involve prepartitioning the design shapes prior to correction for the proximity effect. They employ a set of rules-of-thumb, based on the layout of the design, in order to provide finer partitioning in places where layout of the design indicates that the magnitude of the proximity effect is likely to be large. For example, fine partitioning is used where there is an influence of surrounding shapes on a given shape, and a coarser partitioning is used where there are no surrounding shapes in a vicinity of the given shape.
The simplest such method is described in Grobman, Speth and Chang, "Proximity Correction Enhancements for 1 .mu.m Dense Circuits Magnitude and Correction Techniques" which appeared in IBM J. Res. Dev., Vol. 24, pp. 537-544, (1980), where each sufficiently large rectangle is partitioned into an internal area and four "sleeves" (narrow perimetral rectangular strips). No further partitioning of sleeves is made. This method works well where the size of the smallest design shapes is around 1 .mu.m.
Another method of shape partitioning is described in Parikh, "Technique for Automatic Subdivision of Pattern Data for Enhanced Proximity Effect Correction" which appeared in the IBM Technical Disclosure Bulletin Vol. 24, pp. 438-451, (1980). This method employs the self-consistent technique of proximity correction where the doses, which have to be assigned to prepartitioned shapes, are defined as a solution of a set of linear equations. In this method the partitioning is refined, if needed, on each iteration and then the proximity correction is made once more, until some criteria on the proximity effect variations across each subshape are fulfilled. The method has disadvantages of low computational efficiency and of a need to define the basic prepartitioning with a dense set of predefined sample check points.
The prior art method known as "intelligent partitioning" is described in Kratschmer, "Verification of a proximity effect correction program in Electron beam Lithography", J. Vac. Sci. Tech B, Vol. 19, pp. 1264-1268, (1981). In order to correct for intra-shape proximity effect each sufficiently large rectangle is partitioned into an internal area, four narrow sleeves on the four sides of the rectangle and four small corner squares. Each sleeve is further partitioned by a projection of neighboring shapes onto the current subshape. The design is usually partitioned too fine, so that in the data compaction stage some adjacent rectangles with almost the same dose could merge together. A similar approach is also described in Otto and Griffith "Proximity correction on the AEBLE-150", J. Vac. Sci. Tech. B, Vol. 6, pp. 443-447, (1988).
The disadvantages of intelligent partitioning type methods are firstly high computational load and secondly they produce a greater number of subshapes than are necessary for optimal partitioning. These disadvantages reduce the E-beam lithography throughput.
All the prior art shape partitioning methods partition the design on the basis of the layout of the design shapes. However, there exist methods of proximity correction, such as the method described in Pavkovich "Proximity Effect Correction Calculations by the Integral Equation Approximate Solution Method", J. Vac. Sci. Tech. b, Vol. 4, pp. 159-163, (1986), which use what could be labelled a ""field" approach. In this approach either the backscatter or the dose boost is described by a slowly changing continuous function over the design. This function is obtained, at points on a grid, as a solution of an integral equation. The continuous function given by interpolation of the grid values in these methods acts as an indicator field which contains all the information on the degree or magnitude of the proximity effect. The required dose at any point on the design is found through a predetermined relationship with the indicator field.